Another thing you could do is put a ping-pong ball into the gas tank. At first, it floats in the gas and the car runs fine but eventually, the ball finds its way to the hose that takes the gas to the engine and gets stuck, being too large to actually travel down said hose. This prevents any more gas from getting into that hose and the car stalls because it seems to be out of gas, even though the gauge says the tank is full. Once the car has stopped and probably been turned off, the ball will fall back into the gas tank, floating harmlessly once again...until the car is turned on again and the whole process starts over. Also, the driver won't realize it's sabotage until a mechanic takes the gas tank out and finds the ping-pong ball!
